1. Field of the Invention
The present general inventive concept relates to an audio reproduction system, and more particularly, to a method and apparatus to decode audio matrix in which a moving sound image is restored by using an audio reproducing device such as a digital television (DTV) or audio-video (AV) receiver.
2. Description of the Related Art
Traditionally, when a user wanted to see a movie or the like at home, the user could see, for example, a movie through ground wave broadcasting from a television broadcast, etc. However, recently, the user can listen to an original sound of a movie, etc., due to the spread of video tapes, video discs or satellite broadcasting. In video tapes, video discs and satellite broadcasting in which the user listens to the original sound of the movie, audio signals of a plurality of channels are matrix-processed to be encoded as audio signals of two channels. In addition, when a dedicated decoder is used, audio signals of five channels such as front left (L), center (C), front right (R), left surround (Ls), and right surround (Rs) are restored from audio signals of two channels. Due to center channel signals of the audio signals of five channels, a sense of localization which is definitude of a sound can be obtained, and due to surround channel signals, a sense of presence is improved due to a moving sound, an environment sound, and a remaining sound, etc.
A matrix decoder that has been generally used, generates center channel signals and surround channel signals by using a sum of two channel signals and a difference therebetween. An audio matrix decoder in which matrix characteristics are not changed is well known as a passive matrix decoder. When each channel signal separated by the passive matrix decoder is encoded, audio signals of other channels are scaled-down together with corresponding channel audio signals and are linearly combined. Thus, signals of channels output to a conventional passive matrix decoder have low separation between channels so that localization of a sound image is not clearly achieved in a multichannel environment. An active matrix decoder adaptively changes matrix characteristics so as to improve separation between two-channel matrix symbol type encoding signals.
A technology relating to such matrix decoder is disclosed in U.S. Pat. No. 4,799,260 (filed 6 Feb. 1986, entitled VARIABLE MATRIX DECODER), WO 02/19768 A2 (filed 31 Aug. 2000, entitled METHOD FOR APPARATUS FOR AUDIO MATRIX DECODING).
Referring to FIG. 1, in a conventional matrix decoder, gain function units 110 and 116 clip input signals so as to balance levels of stereo signals Rt and Lt. A passive matrix function unit 120 outputs passive matrix signals from stereo signals R′t and L′t output from the gain function units 110 and 116. A variable gain signals generator 130 generates six control signals gL, gR, gF, gB, gLB, and gRB in response to the passive matrix signals generated in the passive matrix function unit 120. A matrix coefficient generator 132 generates twelve matrix coefficients in response to six control signals generated in the variable gain signals generator 130. An adaptive matrix function unit 114 generates output signals L, C, R, L, Ls, and Rs in response to the input stereo signals R′t and L′t and the matrix coefficients generated by the matrix coefficient generator 132. The variable gain signals generator 130 monitors levels of signals according to channels, calculates an optimum linear coefficient value according to the monitored levels of signals according to channels, and reconfigures multichannel audio signals. The matrix coefficient generator 132 increases a level of a channel having a largest level nonlinearly.
However, in a conventional matrix decoding system illustrated in FIG. 1, a position of a virtual sound source generated in a multichannel environment is not considered. Thus, localization of a sound image is not precisely achieved in a space. Furthermore, precisely representing a change in positions of a sound source moving in a virtual space is not easily accomplished. Thus, a capability of dynamically expressing a sound image is insufficient. That is, the conventional matrix decoding system is not capable of restoring a sound image moving between channels so as to restore surround sound and a sound image that exists in a rear channel (a surround channel).